Under a public key code system, an entity confirms the reliability of information through utilizing a certificate from a certificate authority. The entity attaches the certificate to the information and sends them to another entity.
If a private key of an entity is exposed, or an entity malfunctions, or the access authority of an entity is revoked, the certificate authority needs to revoke the certificate before the certificate expires. To revoke the certificate, the certificate authority utilizes a certificate revocation list (CRL) including a value of the certificate to be revoked and provides the certificate revocation list to the entity, whereby the certificate can be verified.
In some networks, for example, a vehicular network, the certificate authority produces ten millions of certificates to the entity, and the entity can switch the in-use certificate at a frequent basis. Therefore, the privacy and identifiability of the entity can be protected. In such networks, the certificate revocation list needs to list the ten millions of certificates to the entity when a certificate of the entity needs to be revoked. Therefore, the certificate revocation list becomes long and results in a procedural burden to the entity.